The solidification in solution of oxides metals which have a lower oxidation degree (Ca, Mg, rare earths such as Y, Sc, Sm, Pr) than those of certain tetravalent metal oxides (Zr, Ce, Th, Hf), is accompanied by the formation of anionic lacunas, which confer to the solid solution an ionic conductivity extremely selective to oxygen, and therefore this material may be considered as a real solid electrolyte.
Therefore, if the two faces of such a material are exposed to different oxygen pressures, a difference of potential appears. Conversely, the application of an electric current to this material by two external electrodes provokes an actual "pumping" of oxygen through the solid, the flow of which is proportional to the intensity of the current applied.
This property has already been used for oxygen dosing; these devices are called potentiometric sensors, where the response, between 700.degree. and 900.degree. C., is ideal: its precision is 1% between 1 to 10.sup.-8 atm. They are mostly used to control and to regulate the air-petrol mixture in internal combustion engines and in metallurgical ndustries.
However, this relatively low operating temperature range still limits the applications of these materials. Effectively, whereas a lower utilization temperature could be desirable for many uses, it was noted that a temperature decrease results first of all in greater resistance of the material at low temperature and secondly, in a slowing down of the kinetics of the oxido-reduction reactions on the electrode. The result is that below 600.degree. C., the response times of these devices become excessive which causes errors that may be imputed to complex and badly controlled phenomenons between the opposing phases of the electrodes.
Some researchers have tried to compensate for these inconveniences by selecting the materials and by improving the morphology of the electrodes, but they have not yet obtained results that can be used industrially. However, it can be admitted that, during the complex and unknown processes of transferring loads to interfaces between the three opposing phases of the electrodes ("electrolyte" solid solution, metallic electrode and gaseous oxygen), the kinetics of the electrochemical reaction, and as a result the electric current going through the interface, are limited by the slower process, which is linked to the oxygen pressure. In other words, the electrodes-electrolyte-gas interfaces present a resistance to the electric current going through, designated by "electrode resistance", which depends on the oxygen pressure, according to a relation which is a function of numerous parameters, but which, for a given electrodes-electrolyte combination and a given microstructure, is well defined and reproducible.